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Forum: Pentax SLR Lens Discussion
11-12-2014, 11:15 AM
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Focal length is a mathematical formula based on the size and position of lenses and their distance from a convergence point (when focused to infinity).
Super easy version that I *think* is a pretty accurate, if simplified, explanation: a simple "sherlock holmes" style magnifying glass held above a piece of paper at 15 cm or 40 cm might put a big blob of sunlight on the paper, but at 25cm, it will focus to a fine point and eventually start the paper burning. Its focal length is 250mm (25cm). Adding additional lenses with different curves to correct for distortion, or to spread the light back out, or to concentrate the light even further, changes the focal length, because now instead of making a point of light at 25cm, it might make the point at 35cm, or 5cm - that would make its focal length 350 or 50 mm.
So, a 50mm lens is a 50mm lens is a 50mm lens. A 50mm lens from a 6x7 camera is a 50mm lens on the K mount, but if you want it to focus to infinity (or possibly at all) you need the lens to be as far from the K mount camera's sensor or film as it would have been from the 6x7, since that's the distance at which the light converges. (wouldn't it be nifty if someone made a LensTurbo/SpeedBooster style reducer for 67 and 645 lenses? going from a 3920mm^2 sensor to a 372mm^ sensor would be most interesting (and probably unusable lol))
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